Don't confuse the two. All Syty's have a roller cam, meaning roller lifters. The "rockers" can be standard or roller tipped or even full roller. If the cam in your truck is not stock, then there may be the problem using the 1.6s, other members can correct me, but I do believe the pushrod opening in the head is constricted and has to be modified in some cases for high lift situations.
The main advantage of the roller tipped and full roller rockers is to reduce friction and the power it takes to overcome the friction. Adding 1.6 rockers on a stock cam gives a little more lift, though I think the verdict is out on the benefits as some members have had mixed results.
So find out from the previous owner, what cam is in the engine? Hope this helps somewhat.
